There are well known photographic processes for obtaining direct positive images without requiring a reversal processing step or a negative film.
The processes for preparing positive images using conventional silver halide direct positive photographic materials can be chiefly classified into the following two groups from the viewpoint of practical usefulness, except for certain special processes.
The first type is a process using a pre-fogged silver halide emulsion, in which a direct positive image is obtained after development by breaking fog nuclei (latent image) in exposed areas utilizing solarization or the Herschel effect.
The second type is a process using an internal latent image silver halide photographic emulsion which is not previously fogged, in which a direct positive image is obtained by conducting surface development either after fogging processing or while carrying out fogging processing after image exposure.
The term "internal latent image silver halide photographic emulsion" as used herein means a silver halide photographic emulsion which has sensitivity specks chiefly in the interior of silver halide grains, and a latent image is formed chiefly in the interior of the grains by exposure.
Known methods for selectively forming fog nucleui (a latent image) as described above, include a method generally called a "light fogging method", in which a second exposure is given to the whole surface of a sensitive layer (described in, for example, U.K. Patent 1,151,363) and a method called a "chemical fogging method", in which a nucleating agent is used. These methods are described in, for example, Research Disclosure, Vol. 151, No. 15162, pages 76-78 (November 1976).
The chemical fogging process has generally higher sensitivity than the light fogging process, so that the chemical fogging process is suitable for use in fields which require high sensitivity. The present invention relates to a chemical fogging process.
Various techniques in this field are known and described in U.S. Pat. Nos. 2,592,250, 2,466,957, 2,497,875, 2,588,892, 3,317,322, 3,761,266, 3,761,276 and 3,796,577 and U.K. Patents 1,151,363, 1,150,553 and 1,011,062.
Photographic materials having relatively high sensitivity as direct positive materials can be prepared by using these known processes.
The mechanism of forming a direct positive image is described in more detail in James, The Theory of the Photographic Process, (4th ed), Chapter 7, pages 182-193 and U.S. Pat. No. 3,761,276.
Without being limited by theory, it is believed that fog nucleui are selectively formed on the surfaces of silver halide grains in unexposed areas by a surface desensitizing action due to the internal latent image formed in the interior of silver halide grains by first imagewise exposure. A direct positive image is then formed in the unexposed areas by carrying out a surface development treatment.
In forming direct positive images by using the light fogging method or chemical fogging method, the development rate is low and processing time is long as compared with conventional negative materials, although processing time has been shortened by increasing the pH value of the developing solution and/or elevating the temperature thereof. However, when the pH value or the temperature is high, the minimum image density of the resulting direct positive image is generally increased. Further, the developing solution is liable to be deteriorated by air oxidation, when the developing solution is used under high pH conditions. As a result, developing activity is greatly lowered.
As methods for increasing the maximum density of the direct positive image while keeping a low minimum density, there are known a method using hydroquinone derivatives (U.S. Pat. No. 3,227,522) and a method using mercapto compounds having a carboxyl group or sulfo group disclosed in JP-A-60-170843 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). However, the effects obtained by using these compounds are insufficient. An effective method has not been found which can increase the maximum density of a direct positive image without increasing the minimum density thereof. A method is desired which gives a sufficient maximum image density, even when a developing solution having a low pH is used.
In the formation of direct positive images by the light fogging method or the chemical fogging method, there is a problem that gradation is soft. This tendency is particularly remarkable in highlight areas. Thus, images formed by these methods have the disadvantage that tone is liable to be insufficiently reproduced.
As methods for obtaining a photographic material for forming higher contrast direct positive images to solve this problem, there are known a method using a monodisperse system of silver halide grains; a method in which silver halide grains are doped with a polyvalent metal ion (U.S. Pat. Nos. 3,367,778 and 3,287,136); and a method in which a core/shell type emulsion is used and the sensitization of the core part is adjusted (U.S. Pat. No. 4,035,185). However, the effects obtained by these methods are insufficient.
Further, a method using mercapto compounds having a carboxyl or sulfo group (JP-A-60-170843) is known. However, the effects obtained by using these compounds are insufficient, and no conventional method is capable of effectively raising the maximum density of a direct positive image without causing an increase in the minimum density thereof. A method is desired, for obtaining a sufficient maximum image density, even when a developing solution having a low pH is used in particular.